Structure of surface polysaccharides affects symbiotic nitrogen fixation in Frankia alni ACN14a

Frankia spp. are nitrogen-fixing actinobacteria that establish root-nodule symbiosis with actinorhizal plants comprising more than 200 species in eight dicotyledonous families. From a cell population of Frankia alni strain ACN14a that had been subcultured for over 30 years, we identified two types of variants (types A and S) with a different colony appearance. Type A variants exhibited ambiguous, and type S variants exhibited sharp colony edges. The two types differed in molecular weight and monosaccharide composition of cell surface polysaccharides, which could be responsible for the differences in colony appearance. When inoculated to a host plant Alnus glutinosa, both types induced nodulation but plants infected with type S isolates showed much lower nitrogen fixation activity than those infected with type A isolates. Indeed, type S isolates developed fewer vesicles inside infected plant cells. These results suggest that surface polysaccharides of Frankia play important roles in symbiotic interactions with actinorhizal plants.